Nonfood Applications of Proteinaceous Renewable Materials - Journal

Chemistry for Everyone edited by

Products of Chemistry

George B. Kauffman California State University Fresno, CA 93740

Nonfood Applications of Proteinaceous Renewable Materials Justin R. Barone* and Walter F. Schmidt Environmental Management and By-Product Utilization Laboratory, USDA/ARS/ANRI, Beltsville, MD 20705; *[email protected]

It is well known that proteins are abundant in food and are vital to nutrition and biochemical function. What is not well known is that proteins derived from agricultural sources are used in everyday products such as glue and textiles. Research continues to find new uses for proteins in a wide variety of applications, most of which would be replacements for petroleum-derived materials. Proteins can be a viable source of polymers for fiber, molded plastics, films, and an array of products currently supplied by the synthetic polymers industry. The big advantages are that proteins are derived from a sustainable resource and can be processed in much the same way as conventional synthetic polymers. While there are many current and future nonfood uses for proteins, it is the intent of this review to concentrate on recent advances focusing on uses as polymers and biomaterials, which have enormous commercial potential. Historical Nonfood Uses of Proteins The use of proteins in nonfood products dates back at least hundreds of years (1). Historically, glue has been made from animal hide and milk proteins such as collagen and casein, respectively (2). Bovine serum albumin obtained from animal blood is a major component of wood adhesive (3). Gelatin derived from animal bones has been used to encapsulate drugs for the pharmaceutical industry for controlled release (4, 5). Wool fiber is keratin protein from sheep and is easily collected and woven into a variety of fabrics, as is silk, which is silk fibroin protein (6, 7). Leather is composed of collagen from animal hides (8, 9). Collagen and keratin and short chain peptides derived from collagen and keratin are used in cosmetics and hair and skin care products (10). Most of these protein-based products rely on tried-and-true processing techniques developed over decades. For instance, a simple laboratory experiment can be performed in the classroom that can convert milk to casein glue (11). All of these examples represent commercial nonfood uses for proteins. Recent advances in innovative technologies for the use of proteins show uses that are not yet commercial, but could have enormous potential and indicate what the future can hold for agricultural proteins in making value-added products.

leum-based polymers account for about 11% of the 229 million tons of municipal solid waste generated in the United States each year (22). Packaging that is eventually biodegradable is an advantage given this large quantity of synthetic polymer that must be recycled or landfilled, considering typical “tipping” or land-filling fees in the United States are about $50 per ton. Another use is as biomaterials. In this use the biopolymers must be bioresorbable in the long term, meaning that after they perform their function they are degraded and metabolized by the body. It may not be feasible to use agricultural proteins in some biomedical applications if they are not of the highest quality or if prion diseases such as mad cow disease become a bigger issue. However, these applications represent small volumes of polymer, and research using agricultural polymers may yield motifs to utilize the same polymers directly from the human requiring treatment. If the polymer were derived directly from the individual, there would be minimal concern of contamination or of rejection. Processing of agricultural products for food results in protein waste. Currently, some of this protein waste is used in pet food and animal feed. The rest is itself landfilled. For instance, following milk processing, whey protein is left behind that contains large quantities of the protein lactalbumin (15). Similarly, feathers are a large waste problem for the poultry industry and are made entirely of keratin (23). These proteins are polymeric in nature and because they are prevalent in the agricultural waste stream they may be a sustainable alternative to petroleum feedstocks. With petroleum prices over $70 a barrel and land-filling fees about $50 per ton, use of sustainable proteins that degrade may help keep petroleum prices and land-filling fees in check. Basics of Protein Chemistry There are 20 amino acids that occur in nature that arrange themselves in various sequences, called the amino acid sequence, to form all proteins. The general formula of an amino acid where R is the chemical group that defines each specific amino acid (24, 25) is shown in Figure 1. For instance, if R is H the amino acid is glycine or if R is CH3 the amino acid is alanine. It is easy to see why these molecules are called amino (NH2) acids (COOH). Proteins are formed

Current and Future Nonfood Uses of Proteins Currently, proteins derived from soy, whey, peanut, wheat, corn, fish, and feather processing are being used in novel ways for possible use in new value-added products (12– 21). The most prevalent uses being considered are food packaging and biomaterials. Naturally derived proteins would be biodegradable and bioresorbable. Nonbiodegradable petrowww.JCE.DivCHED.org

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H R

C

O C

NH2 OH

Figure 1. The general structure of an amino acid. The group “R” defines the specific amino acid.

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through the formation of peptide bonds by condensation polymerization as shown in Figure 2. The carboxyl group on one amino acid reacts with the amino group on another and a molecule of water is in turn released. The amino acid that is left in the protein is called a “residue”. Proteins are unusual polymers in that the “monomer” or smallest repeatable molecule in the polymer is the amino acid sequence, which usually has a molecular weight in the thousands. Most synH H 2N

H

C

C

R

O

OH

H H 2N

H 2N

C

C

R

O

OH

H

C

C

N

C

C

R

O

H

R

O

OH

+

H 2O

Figure 2. Condensation polymerization reaction to form a peptide bond (25).

thetic polymers and biopolymers such as cellulose have repeat unit molar mass of a few hundred grams per mole at most. The amino acid sequence is important because it defines the specific protein and determines the final properties of the protein. For instance, each amino acid is hydrophilic or hydrophobic so the degree of hydrophilicity of the protein is controlled by how many hydrophilic amino acids are in the sequence. As the condensation polymerization proceeds, the protein chains formed can arrange, for example, into α-helices or β-sheets through intramolecular hydrogen bonding depending on the local environment of the protein. If the protein contains the amino acid cysteine then covalent sulfur–sulfur or cystine bonds can form between adjacent cysteine residues. Proline residues in collagen can be covalently bonded in the presence of naturally occurring enzymes in the body. In turn, the helices or sheets can pack together to form higher length scale structures such as crystals. Finally, hydrogen bonding can also occur between adjacent protein molecules. Proteins have many hierarchies of bonding and structure as shown in Figure 3. Not only is the covalent bonding during the polymerization of the amino acid sequence important to final protein performance, but the degree of secondary bonding and structure formation is important as well. For instance, intermolecular cystine bonds formed in cysteine-containing proteins operate the same way as “cross-links” do in synthetic polymers and provide proteins like keratin with high strength, that is, α-keratin in rhinoceros horn has a tensile strength of over 60 MPa (26). Large quantities of crystallinity formed along the fiber axis during spider silk spinning make silk fibers enormously strong with tensile strengths approaching 1000 MPa (27). In contrast, semicrystalline polypropylene and high-density polyethylene have typical tensile strengths of approximately 40 MPa and 30 MPa, respectively (28). The Chemistry That Allows Proteins To Be Utilized There is no doubt that proteins in the native state are tough and lightweight materials. Many of the structural components found in biology, such as the materials comprising bone, muscle, joint, skin, and hair, are made of structures of protein molecules (26). To utilize these proteins, some of the structure must first be broken down so that a new product can be made. To be effective materials, the final product after processing needs to also retain many of the attributes present in the native structure of the protein, the ones that made it so useful in nature in the first place.

Figure 3. Hierarchy of structures in a proteinaceous material.

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Solubilizing Proteins The easiest way to use proteins in new products is to dissolve them in suitable solvents, form a product, then drive off the solvent. Currently, the most common preparation method of protein films for packaging and biomaterial applications is through the solubilization of the protein. Proteins can be solubilized through the use of relevant solvents at various pHs and temperatures. A great example is the protein zein derived from corn, which can be processed into films and is being considered as a biodegradable packaging material and scaffold material in tissue engineering. α-Zein is the most prevalent form in the protein fraction of corn and con-

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tains little cysteine (29). Therefore, α-zein can be easily dissolved in water-soluble organic solvents. However, the final product formed from solution depends greatly on the solvent used, the temperature of solubilization, and the surface the film was cast on, resulting in various surface morphologies (18, 30). Yoshino et al. have made α-zein films from ethanol and acetone solutions in a few minutes at 50 ⬚C (18). The resulting liquid can then be poured into a dish or mold and the solvent evaporated to form a film. Films formed from room temperature solutions of ethanol have little water resistance but films formed from acetone have good water resistance. A schematic representation of a physical model to explain why perhaps the α-zein properties are different when cast from different solvents (18) is shown in Figure 4. Remember, proteins have hydrophobic and hydrophilic amino acids so are usually mixed hydrophobic and hydrophilic. When cast onto a hydrophobic polyethylene sheet, hydrophobic portions of the α-zein associate with the polyethylene sheet. Yoshino et al. propose that there is bonding between the hydrophobic portions of acetone and hydrophobic amino acids in α-zein. After evaporation of acetone, some hydrophobic amino acids are left on the film surface. Conversely, ethanol contains OH, a hydrophilic moiety, so it is less hydrophobic than acetone. Therefore, there is less bonding between hydrophobic portions of ethanol and hydrophobic amino acids in α-zein. After evaporation of ethanol, more hydrophilic amino acids are left exposed at the film surface and more hydrophobic amino acids are left pointing downward, toward the polyethylene sheet, thus decreasing water resistance. In contrast, Dong et al. prepare solutions by solubilizing zein in ethanol at room temperature and casting on hydrophilic glass petri dishes. The resulting films are water resistant in that they are robust in the presence of cell culture media (30). The zein films of Yoshino et al. (18) and Dong et al. (30) appear to have different morphologies that are a function of processing conditions and illustrate perhaps the complexity and versatility of protein molecules. In many cases, control of the solution pH is important to get a good dilute solution and obtain a proteinaceous material in a useable form. Feather keratin solubility is greatly enhanced at mildly basic conditions (21). Acidic conditions degrade peptide bonds and are generally not preferred (31). In contrast, carbohydrate-based materials (e.g., starches), which are virtually 100% hydrophilic, are less versatile. To impart water resistance properties to carbohydrates requires physically or chemically altering the polysaccharide structure to make its molecular surface more hydrophobic.

Thermal Processing Preparing films from solution is a great method for small batches and for preserving the integrity of the protein. However, on a large scale, preparing protein products from solution may not be economically feasible or environmentally friendly if there are large quantities of solvent, acid, or base involved. Economic feasibility would be dependent on, for instance, redistilling the solvent and then reusing it and environmental friendliness would be dependent on conservation of the solvent and not allowing any into the waste stream after processing. Traditional thermal processing techniques for polymers include some mixing operation, such as extruwww.JCE.DivCHED.org

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sion, internal batch mixing, or milling, and a molding operation, such as injection-molding, compression-molding, and vacuum-molding (32). In each case the key is to get the protein into a processable melt form without compromising the integrity of the protein. In nature, proteins usually exist with large quantities of water bound to them. The water is crucial to holding the protein structure together (6). The water imparts flexibility and toughness to the protein by “plasticizing” it. Figure 5 shows how plasticizing a protein molecule makes it more flexible. The protein molecule is surrounded by “obstacles” to its movement, represented by large dots, which are usually other protein molecules that are entangled with it. The “obstacles” trap the protein molecule in a “tube” defined by the

hydrophilic portion zein molecule hydrophobic portion

hydrophobic sheet acetone solution

ethanol solution

Figure 4. Molecular level view of two possible film structures formed from zein protein cast from acetone solution (left) and ethanol solution (right) onto a hydrophobic polyethylene (PE) sheet. Hydrophobic portions of zein interact with PE sheet on the bottom of the cast film. Casting from acetone leaves hydrophobic portions of zein exposed at top film surface; casting from ethanol leaves hydrophilic portions of zein exposed at film surface (18).

Figure 5. Polymer tube dilation and increase in free volume upon introduction of plasticizer into polymer structure. In this case the plasticizer is water. Larger, dark circles represent the “obstacles”, which are other proteins.

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Redox Reagents If there is excess cross-linking due to cystine bonding in the protein, then some of the sulfur–sulfur bonds (R⫺S⫺S⫺R´) may need to be broken or “reduced” (form1006


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109

10 0

108

10 -1

⑀b

Strain at Break (⑀b)

intersection of the protein molecules (33). Closely packed protein chains form crystals as evidenced by the square. Upon introduction of water (small dots), the water associates closely with the hydrophilic amino acids in the protein and adds free volume to the polymer system by separating polymer molecules or dilating the “tube” (34). There are fewer polymer– polymer molecule intersections per unit volume and the polymer chains move more easily than in the absence of water, making the material more flexible. Loss of that water means the protein has been “denatured”. Remember, water aids in holding portions of the protein structure together through hydrogen bonding. In the absence of water, proteins tend to be brittle materials because of the large quantity of crystallinity and lack of free volume. Proteins become thermally unstable without water and degrade quickly. Thermal processing of polymers typically occurs in the range of 100–300 ⬚C, which would mean that any water in the protein would quickly evaporate. While proteins do not need to be processed at 300 ⬚C, they often process well in the 100–150 ⬚C range (35, 36). The way to do this is to replace the water with another hydrophilic molecule able to “plasticize” the protein above 100 ⬚C and keep it stable long enough to process. Plasticizers that work much like water include hydrophilic, or OH containing, molecules like propylene glycol, ethylene glycol, glycerol, and sorbitol (37). These molecules would replace the water molecule in the protein structure shown in Figure 5. In many cases, glycerol and sorbitol are preferred because they too are agricultural byproducts. Thermally processing proteins with suitable hydrophilic plasticizers is actually quite simple. Typically, the protein and the plasticizer are added in the desired quantities into a polymer-mixing device such as an extruder at a temperature of 100–150 ⬚C. The plasticizer comes into intimate contact with the protein, increasing free volume and making the protein molecules mobile enough to deform them through an extruder die or to mold them into components following extrusion. The specific mechanism with any protein tends to be a little more complicated because the quantity and molar mass of the plasticizer and the properties of the protein polymer greatly affect the properties of the final polymer formulation. Differential scanning calorimetry shows that the glycerol eliminates some of the crystallinity when feather keratin is plasticized. Glycerol has a molar mass of 92 g兾mol as opposed to 18 g兾mol for water. The larger size of the glycerol means that not only does the glycerol dilate the keratin tubes more than water, but it also sufficiently separates the keratin chains to prevent crystallization of keratin molecules (38). Tensile property data for feather keratin processed by plasticizing the keratin with glycerol and sorbitol then compression molding at 160 ⬚C and 10 MPa are shown in Figure 6. Sorbitol has a molar mass of 182 g兾mol and is a crystalline solid at room temperature. Although the sorbitol is a larger molecule, the fact that it crystallizes gives feather keratin materials that are stiffer.

Tensile Modulus (T ) / Pa


T

glycerol sorbitol 107

0

10

20

30

40

50

10 -2 60

Plasticizer Concentration (wt %) Figure 6. Tensile modulus (stiffness), T, and strain at break, ⑀b, of feather keratin blended with various quantities of plasticizer: glycerol (䉭) and sorbitol (䊉).

Keratin Reduction: −

−

R S S R′ + 2H2O + 2e SO3

2−

+ 2 OH−

R SH + HS R′ + 2 OH SO4

2−

+ H2O + 2 e−

Keratin Oxidation: −

R SH + HS R′ + 2 OH SO4

2−

+ H2 O

− 2e− SO3

R S S R′ + 2H2O 2−

+ 2 OH− − 2 e−

Figure 7. Reduction–oxidation (redox) chemistry of sulfur–sulfur bonds in keratin (45).

ing R⫺SH and R´⫺SH) to get a processable polymer. Subsequently, reforming or “re-oxidizing” sulfur–sulfur bonds would restore the intrinsic protein properties. Wool and feather keratin, wheat gluten, lactalbumin from whey protein, and soy protein isolate are all agricultural waste products and all contain fair quantities of cysteine in the amino acid sequence (35, 39–42). Wool keratin has about 11% cysteine, while feather keratin has about 7%, wheat gluten has 3%, lactalbumin has 5%, and soy protein isolate has about 3%. Polymer chemistry shows that a molecule only needs a functionality of 2 to get a fully gelled network (43). Therefore, all of the cysteine-containing proteins have the ability to form fully gelled networks and would require breaking of that network to process the polymers into suitable products. One method to form films from cysteine-containing proteins is to dissolve the proteins in the presence of a redox reagent such as sodium sulfite (21, 44). The sulfur–sulfur bonds are reduced as shown in Figure 7 (45). Films can then be cast as described above. After casting, the sulfur–sulfur bonds reform

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the glycerol兾water兾Na2SO3 solution and then extruded. The effect of Na2SO3 on the viscosity of extruded feather keratin at two different extrusion shear rates, γ, is shown in Figure 9. A minimum is observed in the viscosity at around 3% Na2SO3 relative to the protein fraction. This may mean that about half of the cysteine amino acids in feather keratin form intermolecular S⫺S bonds. The polymer blend obtained has a stiffness of 0.1 GPa and strength of 6 MPa in tension, similar to many grades of low density polyethylene, a high volume commodity synthetic polymer (28, 47). Figure 8. Picture of feather keratin polymer blend being extruded through a 1-inch wide flat die.

4000

Apparent Viscosity / (Pa s)

γ = 200 sⴚ1 γ = 400 sⴚ1

3000

2000

1000

0

1

2

3

4

5

6

7

Na2SO3 in solid fraction (%) Figure 9. Apparent viscosity during extrusion versus sodium sulfite content for feather keratin/glycerol/water/sodium sulfite blends at different shear rates, γ.

as shown in Figure 7. This process is analogous to the permanent waving of hair (46). More recently, Orliac et al. (35) and Barone et al. (47) extruded soy protein and feather keratin, respectively, using sodium sulfite. Both were able to obtain an easily extrudable polymer and the underlying chemistry is the same as Figure 7. A picture of the reduced feather keratin being extruded through a 1-inch wide flat die is shown in Figure 8. The process is actually rather simple and can provide a manner for large-scale processing of cysteine-containing proteins. Sodium sulfite (Na2SO3, M =126.04 g兾mol, d = 2.63 g兾cm3) is dissolved in water. The maximum solubility of sodium sulfite in water is 1:3 (48). The quantity of sodium sulfite needed for maximum S⫺S bond reduction is directly related to the cysteine content. Glycerol (M = 92.1 g兾mol, bp = 290 ⬚C, d = 1.26 g兾cm3) is added to the aqueous solution. Both glycerol and sodium sulfite are FDA-approved materials and routinely used in other unrelated commercial food processing procedures. The soy protein or feather keratin is mixed with www.JCE.DivCHED.org

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Functional Proteins: The Next Frontier The fact that proteins contain many amino acids and that each amino acid has a different functional group, R, means that proteins are enormously versatile molecules. There are many sites to functionalize the protein molecule and therefore produce new and novel materials with greatly enhanced properties. Recent work has focused on taking advantage of the versatility of protein molecules to produce materials for tissue engineering, biotechnology, and to serve as a new source of material for polymers. Tissue engineering is a burgeoning field that involves the cultivation of living cells on a substrate with the hope that those cells will grow into a living tissue capable of metabolic function. The scaffold must be strong enough to hold the cells and porous enough to allow cells to proliferate throughout it to grow in three dimensions. The cells must be able to interact with the substrate in the short term so that the cells can grow. In the long term, the substrate must be able to degrade and then be metabolized by the new growing tissue (49). The synthetic polymers available that can satisfy all of these criteria and also meet biocompatibility requirements are limited. Polymers like polylactic acid, polycaprolactone, polyvinyl alcohol, and copolymers of these have received considerable attention. Natural polymers like chitosan and alginate, both polysaccharides, have also been considered. The problem with most of these materials is that they degrade too fast, they cannot be fashioned into intricate three-dimensional scaffolds to grow tissue, or cells do not have an affinity for them. Proteins may be able to overcome these issues (50– 53). For example, collagen cross-linked with glutaraldehyde was recently used as a substrate to grow bone tissue (53). A zein film prepared from ethanol solution has been used as a substrate to culture human liver cells (30). Cells grown on zein grew to a higher density than on a glass substrate. Wool keratin is another protein that has been considered as a tissue-engineering material (50). It has been found that cells prefer certain amino acid sequences, such as arginine–glycine–aspartic acid or RGD (54). However, it is difficult to find proteins with this specific sequence so efforts are underway to functionalize proteins with RGD. The cystine bond sites are easy to modify in cysteine-containing proteins but side chain NH2, COOH, CONH, OH, and pyrrole rings also offer functional sites. Two recent examples involve functionalizing silk and wool keratin with RGD (51, 52). Each attachment scheme occurs at a different amino acid on the protein. In the case of wool keratin, the researchers reduced all of the sulfur–sulfur bonds in the keratin leaving thiol (SH) groups. They then attached the RGD at the SH sites along the keratin molecule as shown

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R G D S K NH2 +

HOOCCH 2CH2Br

R G D S K N CCH2CH2Br H O

R G D S K N CCH2CH2Br

+

H O

HS

R G D S K N CCH2CH2 H O

S

Figure 10. RGD-modified wool keratin chemistry (52).

in Figure 10. It is found that mouse fibroblast cells have a high affinity for this material over untreated petri dishes or unmodified keratin. In the case of silk, RGD is attached at exposed carboxyl groups on aspartic acid and glutamic acid amino acids through a similar chemistry. It has already been stated that proteins like water. For more demanding applications as polymers or plastics, this could be a detriment to the desired material properties if the polymers are in a humid or aqueous environment. For instance, food packaging cannot allow water permeation into the food. Functionalizing the protein with a hydrophobic molecule could solve this problem. Corn zein reacted with polycaprolactone, a biodegradable hydrophobic polymer, creates a copolymer with increased toughness and water resistance over zein alone (55). The hydrophobicity of feather keratin fiber is improved by grafting methyl methacrylate to the thiol groups on cysteine (56). In this case, the intent is to give the fibers more of a hydrophobic character so that they can be used as a short fiber reinforcement for hydrophobic polymers like polyethylene and polypropylene. Woerdeman et al. recently performed experiments similar to those involving sodium sulfite and soy protein and feather keratin (57). These researchers added a large molar mass, thiol-terminated, molecule to the redox reaction mixture. The large branched molecule is then chemically incorporated at R⫺SH sites on the wheat gluten polymer via oxidation (forming S⫺S bonds). If more than one SH site is present on the added chemical, cross-links between adjacent wheat gluten molecules will occur. Thus chemical treatments have imparted increased strength and toughness to the wheat gluten–thiolterminated molecule copolymers. Conclusions Proteins are a promising source of material for making polymers that can be used for fiber, film, molded products, and biomaterials. The agricultural waste stream provides billions of pounds per year of protein that could be used to make these products. Through simple chemistry and traditional 1008


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polymer processing techniques, protein products can be easily realized. With ever increasing petroleum and energy costs, protein-based polymeric materials would not only be sustainable and environmentally friendly but cost-effective as well. Literature Cited 1. History of Hide Glue. http://www.bjorn.net/history.htm (accessed Apr 2006). 2. History of Adhesives. http://inventors.about.com/library/ inventors/bladhesives.htm (accessed Apr 2006). 3. Hojilla-Evangelista, M. P. J. Am. Oil Chem. Soc. 2002, 79, 1145–1149. 4. Bigi, A.; Borghi, M.; Cojazzi, G.; Fichera, A. M.; Panzavolta, S.; Roveri, N. J. Therm. Anal. Cal. 2000, 61, 451–459. 5. Gelatin. http://www.lsbu.ac.uk/water/hygel.html (accessed Apr 2006). 6. Feughelman, M. Mechanical Properties and Structure of AlphaKeratin Fibres; University of New South Wales Press: Sydney, 1997. 7. Silk Protein Project. http://faculty.washington.edu/yagerp/ silkprojecthome.html (accessed Apr 2006). 8. Thorstensen, T. C. Practical Leather Technology, 3rd ed.; Krieger: Malabar, FL, 1985. 9. Liu, C-K. J. Appl. Pol. Sci. 2002, 87, 1221–1231. 10. Morimura, S.; Nagata, H.; Uemura, Y.; Fahmi, A.; Shigematsu, T.; Kida, K. Process Biochem. 2002, 37, 1403–1412. 11. Glue. http://www.germantownacademy.org/students/us/clubs/ KTK/Glue/Index.htm (accessed Apr 2006). 12. Okamoto, S.; Setagaya-ku, T. Cereal Foods World 1978, 23, 256–262. 13. Kim, K. M.; Marx, D. B.; Weller, C. L.; Hanna, M. A. J. Am. Oil Chem. Soc. 2003, 80, 71–76. 14. Hong, S-I.; Krochta, J. M. J. Food Sci. 2003, 68, 224–228. 15. McHugh, T. H.; Aujard, J-F.; Krochta, J. M. J. Food Sci. 1994, 59, 416–419. 16. Liu, C-C.; Tellez-Garay, A. M.; Castell-Perez, M. E. Lebensm.Wiss. u.-Technol. 2004, 37, 731–738. 17. Mangavel, C.; Barbot, J.; Gueguen, J.; Popineau, Y. J. Agric. Food Chem. 2003, 51, 1447–1452. 18. Yoshino, T.; Isobe, S.; Maekawa, T. J. Am. Oil Chem. Soc. 2002, 79, 345–349. 19. Tillekeratne M.; Easteal, A. J. Pol. Int. 2000, 49, 127–134. 20. Cuq, B.; Gontard, N.; Guilbert, S. Lebensm.-Wiss. u.-Technol. 1999, 32, 107–113. 21. Schrooyen, P. M. M.; Dijkstra, P. J.; Oberthur, R. C.; Bantjes, A.; Feijen, J. J. Agric. Food Chem. 2000, 48, 4326–4334. 22. Municipal Solid Waste. http://www.epa.gov/epaoswer/non-hw/ muncpl/facts.htm (accessed Apr 2006). 23. Parkinson, G. Chem. Eng. 1998, 105, 21. 24. Zubay, G. Biochemistry, 3rd ed.; Wm. C. Brown Publishers: Dubuque, IA, 1993. 25. McQuarrie, D. A.; Rock, P. A. General Chemistry, 2nd ed.; W. H. Freeman and Co.: New York, 1987. 26. Vincent, J. Structural Biomaterials; Princeton University Press: Princeton, NJ, 1990. 27. Elices, M.; Perez-Rigueiro, J.; Plaza, G.; Guinea, G. V. J. Appl. Pol. Sci. 2004, 92, 3537–3541. 28. Fried, J. R. Polymer Science and Technology; Prentice Hall: Upper Saddle River, NJ, 1995. 29. Di Gioia, L.; Cuq, B.; Guilbert, S. Int. J. Biol. Macromol. 1999, 24, 341–350.

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The structures of the amino acids discussed in this article are available in fully manipulable Jmol and Chime format as JCE Featured Molecules in JCE Online (see page 1103).

Featured Molecules

an interactive modeling feature, Only@JCE Online

http://www.JCE.DivCHED.org/JCEWWW/Features/MonthlyMolecules

www.JCE.DivCHED.org

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Vol. 83 No. 7 July 2006

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Nonfood Applications of Proteinaceous Renewable Materials - Journal

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